Why can't I run?!?!

I went for a run today, I intended it to be a 5 mile run it turned into 3.5 with a little walking at the end. I don't know what my problem is I know I can do the distance but today about .5 miles into it my lungs started to hurt. Its not that I haven't warmed up because I did. This happens to me way to often and its starting to make it very hard to train and it makes me feel like I will never be able to accomplish my marathon goal.